This invention relates to velocity measuring, and more particularly to a non-contact velocimeter using photodiode arrays.
There are many known ways of measuring velocity. These prior art velocity measuring systems range from purely mechanical automobile type speedometers to complicated laser or radar devices using doppler or speckle patterns.
For simple velocity measurements such as the speed of an automobile, the purely mechanical speedometer is satisfactory for most purposes. However, there are certain applications where accurate measurements of velocity are required. For example, the military has developed jeep mounted and airborne mounted Position and Azimuth Determining Systems (PADS) which use inertial techniques to determine position. A serious source of error in an inertial system is drift of the accelerometer which can cause large errors if not periodically corrected. The correction for this drift is obtained from knowledge of the velocity of the system. One way of determining the velocity is to stop the vehicle carrying the system so that the velocity becomes zero. While such a procedure is practical but cumbersome with a jeep mounted system, it is obvious that such a procedure is not possible with an airborne system. Further, while a contacting type velocimeter may be used with a jeep, a velocimeter requiring contact with the earth would obviously not be suitable for an airborne system. Thus, contact velocimeters, which generally are not sufficiently accurate for position determining systems, are totally worthless in an airborne system.
Other known types of prior art velocimeters used require that periodic markings be placed on the object being measured. This type of prior art system is often used in paper mills, printing plants and the like. Obviously such a system cannot be used in a position determining system since the ground cannot be marked. Another prior art system that is not practical, particularly for use in vehicle mounted position and azimuth determining systems, are the microwave doppler systems. Microwave doppler systems are generally expensive and are necessarily heavy. Because of the problems and deficiencies of the prior art systems described above, designers of position and azimuth determining systems have benerally selected a laser technique that utilizes the speckle pattern produced when coherent light is reflected from a surface for velocity measurements.
This invention provides a velocimeter that is ideally suited for use in a position and azimuth determining system. The velocimeter of this invention is highly accurate, does not require any mechanical contact with the ground, is simple in design, highly reliable, relatively inexpensive, and is light in weight. Further, as compared to the laser type devices used in prior PADS, the apparatus of this invention does not require a coherent light source, is less expensive and considerably simpler in design. While the apparatus of this invention was specifically designed for use with a position and azimuth determining system, the velocimeter of this invention has general utility particularly in applications where highly accurate velocity measurements are required.